A piezoelectric device using a piezoelectric composition has an effect of generating a deformation when an external electric field is applied and also has an effect of generating electric charges on the surface when it receives an external stress. Recently, such a piezoelectric device is widely used in various fields. For example, a piezoelectric device which uses a lead based piezoelectric composition such as lead zirconate titanate and the like will deform in proportion to the applied electric voltage with the displacement value being a level of order of magnitude of 1×10−10 m/V. Thus, such a piezoelectric device is excellent in fine position adjustment and can be further used for the fine adjustment in an optical system.
As a piezoelectric composition will generate electric charges in proportion to the applied stress or the deformation caused by the stress, it also can be employed as a sensor for detecting minute forces or deformation. Besides, a piezoelectric composition has an excellent responsiveness. Thus, when an alternating current field is applied, the piezoelectric composition itself or an elastomer coupled to the piezoelectric composition can be activated, leading to the occurrence of resonance. In this respect, the piezoelectric composition can also be used as a piezoelectric transformer, an ultrasonic motor or the like.
In general, a piezoelectric composition refers to a kind of substance obtained by providing a polarization treatment to a ferroelectric composition. The polarization treatment refers to a process in which a sintered ferroelectric composition with a random orientation for spontaneous polarization is applied with a direct electric field having an intensity above that of the coercive field of the sintered ferroelectric composition so as to bring the orientation of spontaneous polarization into a specific direction and to provide the ferroelectric composition with a polarity. Most of the ferroelectric composition with the spontaneous polarization and the piezoelectric compositions have a perovskite structure.
The compound with a perovskite structure generally refers to a compound represented by a formula of ABO3. In particular, in such a structure, an alkali metal or an alkali earth metal having a big ionic radius is located in A site and a transition metal ion having a small ionic radius or the like is located in B site while the atom in A site is filled in the gaps in a three-dimensional network of BO6 oxygen octahedral with their vertex being shared.
Now, the piezoelectric composition in application with more than a half having the perovskite structure is widely developed to meet various needs by adding various subcomponents or additives to a lead based piezoelectric composition composed of lead zirconate (PbZrO3: PZ) and lead titanate (PbTiO3: PT). For instance, there are various piezoelectric compositions such as a piezoelectric composition with a low mechanical quality factor (Qm) and a high piezoelectric constant (d33) and a piezoelectric composition with a low piezoelectric constant (d33) and a high mechanical quality factor (Qm). The previous one is used in an actuator or the like for position adjustment which seeks a big displacement via a direct current related method. The latter one is applicable to alternating-current related uses. For example, the latter one is used in an ultrasonic wave-generating device such as an ultrasonic motor.
There are substances other than PZT that can be used as a piezoelectric composition, most of which are substantially solid solutions using the lead based perovskite such as lead magnesio-niobate (Pb(Mg1/3Nb2/3)O3) as the main component.
However, these lead based piezoelectric compositions contain about 60 to 70 wt % of lead oxides having a low melting point and these lead oxides (PbO) is likely to volatilize during the firing process. Thus, if the influence on the environment is considered, less lead oxides is expected to be used. Besides, more and more lead based piezoelectric compositions are predicted to be used if the piezoelectric ceramics and the piezoelectric monocrystal are to be applied to more fields. Also, lead sulfate generated in the chemical reaction between the acid rain and lead are melted and precipitated in the soil, which may pose a threat to the ecosystem. In this respect, a piezoelectric composition free of lead has become an extremely important technical issue.
As a lead-free piezoelectric composition, for example, the barium titanate (BaTiO3: BT), the bismuth layered ferroelectric and the like are well known. However, BT has a curie point (Tc) as low as 120° C. As the piezoelectric property will disappear at a temperature higher than 120° C., it can not be actually used if it is considered to be coupled by welding or used in vehicles. On the other hand, although the bismuth layered ferroelectric has a Tc as high as 400° C. or even higher and is excellent in thermal stability, the anisotropy of the crystal is high. Thus, it is difficult to provide d33 as excellent as that of the lead based piezoelectric composition just through a normal ceramic related process. If an excellent d33 is to be obtained, it will be necessary to make spontaneous polarization oriented through a hot-press method or a hot forging process. In this respect, a problem rises concerning the productivity.
Recently, the sodium bismuth titanate (Bi0.5Na0.5TiO3: BNT) based composition can be presented as a new desired piezoelectric composition having a good d33 and a Tc which is relatively high among the piezoelectric compositions without lead. For example, Reference 1 has disclosed a piezoelectric composition in which BT and calcium titanate (CaTiO3: CT) are added into BNT.